Problem: $ D = \left[\begin{array}{rrr}4 & 0 & 2 \\ -1 & -1 & 2\end{array}\right]$ $ F = \left[\begin{array}{r}-1 \\ 0 \\ -1\end{array}\right]$ Is $ D F$ defined?
Explanation: In order for multiplication of two matrices to be defined, the two inner dimensions must be equal. If the two matrices have dimensions $( m \times  n)$ and $( p \times q)$ , then $ n$ (number of columns in the first matrix) must equal $ p$ (number of rows in the second matrix) for their product to be defined. How many columns does the first matrix, $ D$ , have? How many rows does the second matrix, $ F$ , have? Since $ D$ has the same number of columns (3) as $ F$ has rows (3), $ D F$ is defined.